The Abstract of the following Patent Literature 1, which relates to a technology of weak field control for a permanent magnet synchronous motor, recites “To smoothly switch between vector control featuring a current command calculation unit and voltage phase operation type weak field control, and to improve the efficiency in switching” and “The problem is solved by any one of followings or combination of them. 1. A step is provided for reducing a voltage phase acquired from intermediate voltage command values Vdc* and Vqc* when switching from normal control to voltage phase operation type weak field control. 2. A value acquired by averaging d-axis current detection values is taken as a first d-axis current command value to be input in a d-axis current command calculation unit when returning to the normal control from the voltage phase operation type weak field control. 3. The gain of the d-axis current command calculation unit is switched when returning to the normal control from the voltage phase operation type weak field control. 4. A motor constant (resistance, inductance, power generation constant) identification calculation unit is provided in addition to the normal control and voltage phase operation type weak field control”.